The present invention relates to a disc device which rotates a disc-like replaceable recording medium at a high speed so as to carry out at least reproduction of data, such as, a disc removable type disc device for CD-ROM, DVD, MO, removable HDD or the like, and in particular to a disc device which is effective at a high rotational speed.
A conventional CD-ROM device is composed of a rotary system for rotating a disc, and a pick-up system for reading data from the disc. On the disc, spiral or concentric recording pits are formed, onto which laser is projected from the pick-up system which is driven radially of the disc, and data are read through the reflection thereupon. The radial recording pitches of these recording pits are very fine, that is, 1.6 .mu.m, and highly precise positioning is required between the disc and the pick-up.
The following two problems are main factors which cause hindrance to positioning accuracy.
(1) Vibration caused by the spindle drive system and the pick-up drive system; and PA1 (2) External disturbance exerted from the outside.
The typical those of the problems stated in (1) are unbalance vibration of a rotary system, electromagnetic vibration of a motor or drive reaction of a pick-up drive system. It is most important to ensure a required degree of positioning accuracy under the presence of these factors when designing the device. In particular, the unbalance vibration of the rotary system have been being materialized since the speed of rotation of a disc has been rapidly increased due to requirement of high speed data transfer. Accordingly, as disclosed in Japanese Laid-Open Patent No. H3-86968, balance correction using fluid has been proposed.
It has been desired to increase the data transfer speed of the disc device due to the materialization of multi-media which concerns a large capacity of data of image or motion picture. The higher the rotational speed of a disc in a disc device, the higher the data transfer speed. Accordingly, these years, the rotational speed of an the disc has been being rapidly increased.
The most serious problem which should be overcome for increasing the rotational speed of a disc, is an increase in unbalance vibration. Different from a hard disc device, an optical disc device for CD-ROM or the like is essential in view of the compatibility of a disc as a recording medium used therein. Since the disc is mass-producible by pressing, the manufacturing accuracy cannot be enhanced greatly, and accordingly, unevenness in thickness and eccentricity between the outer periphery and the inner periphery of a disc are large. Further, unbalance in weight caused by printing letters and a pattern on the disc is not negligible.
If a system having such a large unbalance is rotated, unbalance vibration would occurs due to a cause such that the center of gravity of the rotary system does not precisely align with the rotating center thereof. The force causing unbalance vibration is exhibited by the following formula (1): EQU F=m.times..epsilon..times..omega..sup.2 (1)
Where .epsilon. is a distance between the center of gravity and the rotating center of the rotary system, m is a mass of a rotary body, and .omega. is a rotational speed (rotational frequency).
As understood from formula (1), the unbalance vibration is proportional to the square of the rotational speed, and accordingly, it becomes serious rapidly if the rotational speed increases. The unbalance vibration not only vibrate the pick-up system so as to hinder the read/write of signals and to cause noise, but also vibrates the system itself to which the device is attached, thereby remarkably lowering the reliability of computer system itself.
This is because of the higher speed rotation of a disc due to the higher speed data transfer. For example, in the case of an 8-X CD-ROM drive, the rotational speed is 4,200 r.p.m, and accordingly, the problem of unbalance vibration of a disc, as mentioned above, has been being materialized.